title: Calcium permeation of the turtle hair cell mechanotransducer channel and its relation to the composition of endolymph.
abstract: 1. Recordings of mechanoelectrical transducer currents were combined with calcium imaging of hair bundles in turtle auditory hair cells located near the high-frequency end of the cochlea. The external face of the hair bundles was perfused with a range of Ca2+ concentrations to study the quantitative relationship between Ca2+ influx and transducer adaptation. 2. With Na+ as the monovalent ion, the peak amplitude of the transducer current decreased monotonically as the external [Ca2+] was raised from 25 microns to 20 mm. When Na+ was replaced with the impermeant Tris the transducer current increased with external [Ca2+]. These results indicate that Ca2+ can both permeate and block the transducer channels. The Ca2+ concentration for half-block of the monovalent current was 1 mm. 3. To quantify the Ca2+ influx, the fraction of transducer current carried by Ca2+ was measured using the change in bundle fluorescence in cells loaded with 1 mm Calcium Green-1. The fluorescence change was calibrated by substituting an impermeable monovalent ion to render Ca2+ the sole charge carrier. 4. In the presence of Na+, the fractional Ca2+ current was approximately 10% in 50 microns Ca2+, a concentration similar to that in endolymph, which bathes the hair bundles in vivo. The amount of Ca2+ entering was dependent on the identity of the monovalent ion, and was larger with K+, suggesting that the transducer channel is a multi-ion pore. 5. Over a range of ionic conditions, the rate of transducer adaptation was proportional to Ca2+ influx indicating that adaptation is driven by a rise in intracellular [Ca2+]. 6. Shifts in the current-displacement function along the displacement axis in different external Ca2+ concentrations were predictable from variation in the resting Ca2+ influx. We suggest that changes in the resting open probability of the transducer channels adjust the entry of Ca2+ to keep its concentration constant at an internal site. 7. The results demonstrate that endolymph containing high K+, 50 microns Ca2+ and low Mg2+ concentrations, maximizes the transducer current while still allowing sufficient Ca2+ entry to drive adaptation. The hair cell mechanotransducer channel, in its permeation and block by Ca2+, shows behaviour similar to the voltage-gated Ca2+ channel and the cyclic nucleotide-gated channel.
authors.affiliation: Department of Neurophysiology, University of Wisconsin Medical School, Madison 53706, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Ricci
authors.forename: R
authors.initials: R
authors.lastname: Fettiplace
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003051:Cochlea / Q000502:physiology; D004558:Electric Stimulation; D004710:Endolymph / Q000378:metabolism*; D006198:Hair Cells, Auditory / Q000378:metabolism*; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D008465:Mechanoreceptors / Q000378:metabolism*; D008839:Microelectrodes; D011188:Potassium / Q000378:metabolism; D051381:Rats; D015398:Signal Transduction / Q000502:physiology*; D012964:Sodium / Q000502:physiology; D013050:Spectrometry, Fluorescence; D014426:Turtles / Q000502:physiology*
chemical_list: D007473:Ion Channels; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D002118:Calcium